Grid-pulsed cavity oscillator



@a @.9 L 'Uw y@ SM5@ mom Sept' 27, 1949' c. E. DOLBERG 2,483,337

GRID-PULSED CAVITY OSCILLATOR Filed NOV. 27, 1943 F7471?. F753 Z Patented Sept. 27, 1949 GRID-PULSED CAVITY OSCILLATOR Charles E. Dolberg, Philadelphia, Pa., assignor, by mesne assignments, to Philco Corporation, Philadelphla, Pa., a corporation of Pennsylvania Application November 27, 1943, Serial No. 511,985

(Cl. Z50-36) 7 Claims.

This invention relates to a method and means for improving the operation of cavity-type oscillators. The invention further relates to a method and means yfor controlling the operation of such oscillators through the agency of a control signal applied to the control grid of the oscillator tube. More particularly the invention relates t a method and means for grid-pulsing .an oscillator of the type comprising a lighthouse tube, or the like, and resonant cavity.

An important component of radio detecting land ranging equipment (radar) is the source of high frequency pulse signals (interrupted continuous waves) In a typical radar system the pulse duration may be of the order of l microsecond, and the pulses may occur at the rate of 1000 per second, there being an interval of about 1000 microseconds between pulses. The carrier frequency is usually in the ultra-high frequency range, the wave length being measured in terms of centimeters rather than meters. At these frequencies the carrier is most conveniently generated by means of a vacuum tube operatively associated with a suitable cavity resonator.

I-Ieretofore it has been customary to operate oscillators of this character in accordance with the known plate-pulsing method, wherein the high voltage plate potential is applied to the tube only during the brief pulse intervals during which carrier energy is to be generated. While the present invention is applicable to such platepulsed systems, and can be used to improve the operation thereof, the invention is particular-ly useful .as a means to make practical the gridpulsed operation of cavity oscillators, a mode of operation which has heretofore met with little or no success. According to the grid-pulsing method the plate of the Ivacuum tube is connected directly to a high voltage sour-ce of plate current. The oscillation of the tube is controlled by applying to the control grid thereof a control signal which renders the tube operative, that is, oscillating, only during the brief pulse intervals. In the longer intervals between pulses the grid is biased to a negative potential of such magnitude that the tube is in the condition of plate current cutofi.

One of the factors which has been detrimental to the `operation of both the plate-pulsed and grid-pulsed cavity oscillators has been the presence of spurious oscillations within the cavity. These spurious oscillations were found to be particularly strong an-d troublesome in oscillators of the plate-pulsed variety. By the present innate fthe generation of these spurious oscillations.

It is accordingly a principal object of the present invention to provide a high frequency cavitytype oscillator which, in operation, is substantially free of spurious oscillations.

Another object of the invention is to provide a novel arrangement whereby a cavity-type oscillator may be pulsed through the agency of a control signal applied directly to the control grid thereof.

It is a further object of the invention to provide a method and means for grid-pulsing an oscillator of the type employing ya lighthouse tube in combination with a resonant cavity.

Other features land objects of the invention will be understood by reference to the accompanying drawing, in which Fig. 1 is a schematic diagram of a grid-pulsed cavity oscillator embodying the present invention;

Fig. 2 is an assembly View, partly in section, of a cavity oscillator, illustrating one embodiment of the invention; 4

Fig. 3 is fragmentary detail view of ya portion of the oscillator, showing particularly the grid input structure;

Fig. 4 illustrates an alternative form of a grid input structure which is applicable to the cavity resonator of Fig. 2; and

Fig. 5 illustrates the application of the invention toa cavity oscillator of the plate-pulsed type.

The present invention will best be understood by referring first to the schematic diagram of Fig. l, which illustrates a grid-pulsed ultra-highfrequency cavity oscillatorof a type embodying certain of the features of the present invention. The cavity oscillator l comprises a vacuum tube, preferably of the triode variety, mounted in a suitable resonant cavity 2. The triode may be of the type known as a lighthouse tube (e. g. type 464-A) in which the active portions of the cath-ode 3, the grid 4, and the plate 5 are arranged -in closely adjacent parallel planes, and are generally circular or disk-like in form. Inside the cavity 2 there may be provided .a suitable grid cylinder 6 which is physically supported by the peripheral portion of the grid 4, and electrically connected thereto. An RF plate-choke and cavity tuning member 1 may be associated with the central conductor -or plate pillar 8 which extends concentrically wi-thin the cavity 2 to the plate electrode 5. The cathode 3 is electrically connected to the base of the cavity as shown. The ocsillator l provides a UHF oscillation, the

vention means are provided which largely elimifrequency of which depends upon the design and the dimensions of the cavity. An output signal may be derived by inserting into the cavity a suitable probe 9 in a manner well understood in the art.

Plate voltage may be supplied to the cavity oscillator from a suitable high voltage source, such for example as the source designated +3000 V. In accordance with the grid-pulsing mode of operation, the triode is preferably :biased well below plate-current cut-olf -by the application to the grid I of a high negative bias, supplied in this case from the source designated -175 v.

rIhe apparatus shown to the left of the cavity resonator I comprises means for supplying keying pulses to the grid circuit of the oscillator triode of suicient magnitude to render the oscillator operative throughout the pulse period. It has been found, for reasons hereinafter explained, that it is of the utmost importance that the source of pulse signals have a low effective impedance. Accordingly, pulses may be initially derived from a conventional high impedance source I of pulse signals and applied to a vacuum tube amplifier II. The amplifier II is preferably of the power pentode variety, and has in its output circuit a step-down transformer I2, the secondary winding of which is connected to the -control grid of the oscillator triode. The transformer I2 is preferably of the step-down variety so that the output impedance of the tube II is electively reduced by a factor of 1/N2 Where N is the step-down ratio of the transformer. In a physical embodiment of the invention a step-down ratio of 2 to 1 was employed which effectively reduced the impedance of the pulse source by a factor of 1/4.

It will be understood, of course, that the invention is not limited to the use of the particular low impedance pulse source illustrated in Fig. 1. Various alternative low impedance sources may be employed if desired, such as that provided by a conventional cathode-follower output stage. (See Ultra-High Frequency rTechniques, Brainerd, Koehler, Reich & Woodru; D. Van Nostrand 1942; page 221.)

The low impedance of the pulse source is important for a number of reasons. The source must, of course, be able to supply the high grid current that flows during oscillation, and this is most easily done if the source has a low internal impedance. Moreover, it is of importance that the impedance of the source be low, so that the ilow of grid current which inevitably accompanies the application to the grid of a positive pulse, will not tend to drive the grid back toward its negative condition, and thus limit the magnitude of the plate current during the oscillating period. Also, through the use of a low impedance source, such shunt capacity as is necessarily associated with the pulse input circuit will have only a negligible effect upon the shape of the grid pulsing signal. Finally, it is important to provide a low impedance path to ground in order to decrease the possibility of damage to the tube caused by arcing from the plate to grid, an occurrence which is not at all infrequent. Should such an arc occur, it is desirable that it be limited to a path between the plate and the grid. However, if the impedance from the grid to the cathode is high (as would be the case with a high impedance pulse source) a secondary arc may be established between the grid and cathode. When this occurs there is usually accompanying damage to the active coating of the cathode. It has also been found that in the event of spark-over from plate to grid, the grid-to-ground insulation is less subject to damage if the pulse source provides a low impedance leakage path from grid to ground. The arc itself may be extinguished by means of the circuits described in the copending application of Richard G. Clapp, Serial No. 508,731, led November 2, 1943, now U. S. Patent No. 2,428,616.

By means of the circuit illustrated in Fig. l, and by using a pentode type 6AG7 pulse amplifier driven into the region of plate current saturation, it was possible in one physical ernbodiment of the circuit, to reduce the output impedance of the pulse supplying circuit to approximately 250 ohms.

In the plate-pulsed Version of the cavity oscillator employing tubes of the lighthouse variety, it is customary to connect the grid directly to the outer cavity cylinder 2 at one or more points in the plane of the grid 4. Where, however, it is desired to control the operation of the cavity oscillator through the agency of a control signal applied to the grid, it is, of course, impossible to effect such a direct connection between the grid il and the cavity 2. The present invention provides suitable means whereby the pulse signal derived from the transformer I2 may be passed through the wall of the cavity 2 and applied to the grid 4, without introducing electrical circuit factors which militate against the proper operation of the oscillating system.

It was found that in applying the control pulse to the grid 4 it was necessary to satisfy two major conditions. First, the operating RF impedance path from the lighthouse tube grid cylinder to the shell of the cavity must remain substantially undisturbed, i. e., low. Second, the path from the lighthouse tube grid to the outer shell of the cavity, for currents of the frequencies which are represented in the grid pulsing signal, must be of high impedance compared to the impedance of the pulse source. Two methods of achieving these ends are illustrated in Figs. 2 to 4 of the accompanying drawings.

The first method is illustrated in Figs. 2 and 3. Fig. 2 is an assembly View, partly in section, of a cavity oscillator embodying the present invention; while Fig. 3 is a detail View showing the structure provided by the invention. The oscillator comprises a lighthouse tube I3 to which is clamped, by means of a suitable clamping arrangement I4, an outer cavity shell 2. As will be understood by those skilled in the art the lighthouse tube I3 comprises a grid (not shown) which lies in the plane of the outer grid disc or ring I5, an anode structure I6, a portion (not shown) of which extends to within a few thousandths of an inch of the grid plane, and the cathode (not shown) which extends from the base portion of the tube to within a few thousandths of an inch of the grid. The grid cylinder 6, which corresponds to the similarly designated element of Fig. l, is mechanically fixed and electrically connected to the grid ring I5, as illustrated. The plate pillar 8, which corresponds to the similarly designated element of Fig. 1, frictionally engages the plate cap of the lighthouse tube and extends therefrom, concentrically of the outer cylinder 2, through an insulating end-plate I8 positioned in the upper end of the outer cylinder 2. A cavity tuning member and RF choke I9 is mounted on the plate pillar, and the position of this device with respect to the longitudinal axis of the cavity determines the effective length, and hence affects the resonant frequency, of the cavity. The RF output signal may be derived from the cavity by means of a suitable probe structure I1.

As has been indicated above, in the plate-pulsed operation of cavity oscillators of the type herein dealt with, the grid cylinder 6, and hence the grid and grid ring l5, is connected directly to the outer cavity cylinder 2 by means of one or more connectors, e. g., machine screws, which lie in the plane of the grid ring l5. In the gridpulsed mode of operation, however, it is necessary that the pulse signal be applied between cathode and grid. However, since the direct RF connection between the grid ring l5 and the outer cylinder 2 must be preserved, the special grid input structure including the member 20 has been provided. This structure, as shown in Fig. 3, comprises a threaded sleeve 2l which is soldered to the outer cylinder 2, and which extends through the cylinder 2 for approximately one-half the distance between the said cylinder and the grid cylinder 6. This sleeve is conveniently of brass. Inside the threaded sleeve 2l, and coaxially aligned therewith, is a solid brass rod 22, internally threaded at its inner end, where it engages the contact screw 23 which is soldered to the grid cylinder 6. This connection is preferably, but not necessarily, in the plane of the grid of the lighthouse tube. More specifically, the grid connection should be made at a location in the cavity which is at low or minimum voltage with respect to the operating frequency. This location may be found experimentally by inserting a probe, connected to a suitable detecting means, through a longitudinal slot in the cavity wall.

Between the axial conductor 22 and the outer sleeve 2l is a suitable dielectric, such as the polystyrene sleeve 2li, which insulates the axial conductor 22 from the sleeve 2l. The dimensions of the hereinbefore described structures are not critical. It is important, however, that the capacitance between the axial conductor 22 and the sleeve 2l shall be suicient to provide a lowimpedance by-pass for the generated carrier frequency. This capacitance must not, however, be so large as to introduce a by-passing effect on the signal source at pulse frequencies, since to do so would deleteriously affect the pulse shape. The capacitance afforded between the axial conductor 22 and the sleeve 2| offers a low RF impedance path from the grid to the outer shell of the cavity at the operating frequency, and also is a by-pass for spurious frequencies which may be generated in the cavity. Since the grid is a low voltage point as far as the operating RF is concerned, the characteristic RF eld is not disturbed by the presence of the grid input structure at this point.

In the construction of the grid input elements above described, it is convenient to slot the outer threaded portion of the sleeve after the manner of a collet chuck, and to provide a cooperating locking nut 25 which may be tightened down on the sleeve to bring the slotted portions of the sleeve into firm engagement with the insulating sleeve 24 which, in turn, is caused firmly to grasp the axial conductor 22.

A second method of grid-pulsing the tube is illustrated in Fig. 4. Since the outer shell member 2 and the grid cylinder 6 are the same, only those portions of the structure which embody essential differences are illustrated in Fig. 4. An axial conductor 26 of relatively small diameter extends from a point outside the cavity 2, through an opening therein, to a point on the grid cylinder 6, which point preferably is approximately in the plane of the grid ring I5. The axial conductor 26 carries a quarter-wave RF choke 2l having its outermost closed end 28 positioned at a distance d from the grid cylinder. In general the distance d may be any integral multiple of a half wave length, preferably one-half wave length. In practice the distance d may be somewhat less than this due to the series inductance of the inner conductor 26. It will be understood, of course, that where wavelengths, or fractions thereof, are mentioned, reference is had to the wavelength in the particular dielectric medium or media involved, and not necessarily to the wavelength in air. Soldered to the outer cylinder 2 is a hollow metal tube 29 which extends somewhat beyond the end 28 of the quarter-wave choke 2l. A dielectric sleeve 3U is introduced between the tube 29 and the quarter-wave choke 21. The -capacitance introduced between the choke and the tube 29 provides, with the inductance of the RF choke, a complete filter section. The connection to the grid cylinder may be made at the same point as in the rst described method, i. e. at a point of minimum RF potential.

Although it is not felt that the operation of the grid input circuit of Fig. 4 is understood in all its details, it is believed that, at the optimum adjustment of the quarter-wave choke, the inductance of the inner conductor 26 series resonates with the choke-to-cavity capacity, and thus reduces the impedance between grid and cavity cylinder 2 to a very low value at the operating frequency. At frequencies lower than the operating frequency, where spurious oscillations have heretofore been troublesome, the circuit introduces capacitive reactance between the grid and the cavity cylinder 2. The introduction of capacitive reactance at this point appears to destroy the feedback relations necessary to sustain the spurious oscillations, and in consequence such oscillations are eliminated or suppressed.

By the use of either of the methods described above it is possible to construct grid-pulsed cavity oscillators which are substantially free of spurious oscillations. Spurious oscillations were not observed in any strength in experimental models embodying these constructions. This is due, firstly, to the fact that the input signal is applied at a point of minimum RF potential of the operating frequency, and, secondly, to the fact that the capacitance of the input electrode to the shell of the cavity by-passes any RF voltage that tends to develop at that point.

Although the constructions illustrated in Figs. 3 and 4 are primarily adapted for use with gridpulsed cavity oscillators, it has been found that the operation of plate-pulsed cavity oscillators may be substantially improved through the use of these structures. As has already been indicated, in the plate-pulsed type of cavity oscillator employing lighthouse tubes, it has been customary, heretofore, to connect the grid cylinder directly to the outer cylinder at one or more points in the plane of the grid, by means of a corresponding number of machine screw connections, or the like. It has been found, however, that such direct connection of the grid cylinder to the outer cylinder sometimes gives rise to spurious frequencies which tend to reduce the output of the oscillator at the desired frequency. It has been found that, by substituting for the direct connections a system such as that illustrated in Fig. 5, these spurious oscillations can be eliminated, or at least greatly reduced in strength. In the structure of Fig. 5 there is provided a hollow sleeve member 32 which passes through the outer cylinder 2 of the resonator. This sleeve 32 corresponds to the threaded sleeve 2l of Fig. 3. An axial rod 33 passes through the hollow cylindrical member 32 where it connects to the grid cylinder at a point in the plane of the grid of the lighthouse tube as hereinbefore explained. Since, in the plate-pulsed mode of operation, no external signal is applied to the control grid, the outer end of the axial rod 33 is connected directly to the outer shell of the cavity by means of a suitable conductor 34 which may be regarded as a grid leak, since it provides a path for direct current between the grid and cathode of the lighthouse tube. This conductor may comprise simply a short length of hook-up wire, or it may comprise a small resistance element of the order, say, of 100 ohms.

While preferred embodiments of the invention have been described and illustrated, it will be understood that the invention is susceptible of various modifications, and that the invention contemplates such modications.

I claim:

l. In a grid-pulsed cavity-type oscillator, comprising a resonant cavity, a vacuum tube mounted therein and includingr a grid electrode, a grid cylinder electrically connected to said grid electrode and surrounding the same, said cavity having a wall opening in the vicinity of said grid cylinder, a signal input conductor extending through said opening and electrically connected to said grid cylinder at a point of low voltage with respect to the operating frequency, a hollow conductive member surrounding said conductor in spaced relation thereto and electrically connected to the cavity wall about said opening, and dielectric means interposed between said conductor and said hollow conductive member, the said hollow member and dielectric means being constructed and arranged to provide predetermined capacitance between said conductor and said hollow member, said capacitance being of 4suilcient magnitude to provide a low-impedance path at the generated frequency of the oscillator but of insufficient magnitude to by-pass the relatively lower frequency input signal.

2. In a grid-pulsed cavity-type oscillator, comprising a resonant cavity, a vacuum tube mounted therein and including a grid electrode, a grid cylinder electrically connected to said grid electrode and surrounding the same, said cavity having a wall opening in the vicinity of said grid cylinder, a signal input conductor extending through said opening and electrically connected to said grid cylinder at a point of low voltage with respect to the operating frequency, a hollow conductive member surrounding said conductor in spaced relation thereto and electrically connected to the cavity wall about said opening, said hollow member extending through said opening and having portions disposed interiorly and exteriorly of said cavity, and dielectric means interposed between said conductor and said hollow conductive member, the said hollow member and dielectric means being constructed and arranged to provide predetermined capacitance between said conductor and said hollow member, said capacitance being of suflicient magnitude to provide a low-impedance path at the generated frequency of the oscillator but of insuilcient magnitude to by-pass the relatively lower frequency input signal.

3. In a grid-pulsed cavity-type oscillator, comprising a resonant cavity, a vacuum tube mounted therein and including a grid electrode, said cavity having a wall opening in the vicinity of said grid electrode, a signal input conductor extending through said opening and electrically connected to said grid electrode, a high frequency choke element carried by said conductor and extending through said opening, said choke element having an electrical length equal to a quarter-wave length, a hollow conductive member electrically connected to the cavity wall about said opening and extending exteriorly of said cavity in spaced relation to said conductor and said choke element, and dielectric means separating said hollow member from said conductor and said choke element.

4. In a grid-pulsed cavity-type oscillator, comprising a resonant cavity, a vacuum tube mounted therein and including a grid electrode, a grid cylinder electrically connected to said grid electrode, said cavity having a wall opening in the vicinity of said grid cylinder, a signal input conductor extending through said opening and electrically connected to said grid cylinder, a high frequency choke element carried by said conductor and extending through said opening, said choke element having an electrical length equal to a quarter-wave length, the outer end of said choke element being positioned at a distance from said grid cylinder equal to an integral number of half wave lengths, a hollow conductive member electrically connected to the cavity wall about said opening and extending exteriorly of said cavity in spaced relation to said conductor and said choke element, and dielectric means separating said hollow member from said conductor and said choke element.

5. In a plate-pulsed cavity-type oscillator, a resonant cavity, a vacuum tube mounted thereir. and including a grid electrode, said cavity having a wall opening in the vicinity of said grid electrode, a conductor extending through said opening and electrically connected to said grid electrode, a hollow conductive member extending through said opening and electrically connected to the cavity wall, dielectric means interposed between said conductor and said hollow conductive member, and means electrically connecting said conductor to the cavity wall.

6. In a cavity-type oscillator comprising a resonant cavity, a vacuum tube mounted therein and including a grid electrode, a conductor extending into said cavity through an opening therein, there being a predetermined capacity between said cavity and said conductor, a connection between the inner end of said conductor and said electrode, the self-inductance of said conductor being series resonant with said predetermined capacity at the operating frequency of said oscillator, and an external circuit connecting said conductor and said cavity.

'7. In a grid-pulsed cavity-type oscillator, comprising a resonant cavity, a vacuum tube mounted therein and including a grid electrode, said cavity having a wall opening in the immediate vicinity of said grid electrode, a signal input conductor extending through said opening and electrically connected to said grid electrode, and means for substantially eliminating the generation of spurious oscillations in said oscillator, said means including a hollow conductive member surrounding said conductor in spaced relation thereto and electrically connected to the cavity wall about said opening, and a dielectric member interposed between said conductor and said conductive member, said members being adapted to provide predetermined capacitance between said conductor and said conductive member of sufcient magnitude to provide a low-impedance path at the generated frequency of the oscillator but of insufcient magnitude to by-pass the relatively lower frequency input signal.

CHARLES E. DOLBERG.

REFERENCES CITED The following references are of record in the file of this patent:

Number 10 UNITED STATES PATENTS Name Date George Oct. 1, 1940 McArthur May 26, 1942 Gurewitsch Nov. 19, 1946 Bailey Feb. 18, 1947 

